A VFD operates as a flat triode valve. Thermal electrons emitted from the heated filaments flow through the grid mesh and strike the anode pattern causing luminescence of the display pattern.

Basic VFD Driving circuit

The grid controls electron flow. If a positive potential is applied to the grid, electrons are accelerated towards the anodes. Applying an anode voltage will then turn the display segment on. To ensure uniform brightness across the display, an alternating voltage is used. The waveform is not critical but the rms. voltage applied must be equal to the specified value. High-energy thermal electrons can overcome the potential barrier when the grid voltage is turned off, resulting in leakage luminance. To avoid this, a cut off bias voltage Ek is applied to the filament voltage Ef.